Compositions containing a liquid-crystalline material and a small amount of a dichroic dye are used for production of liquid crystal display elements of a guest-host type (abbreviated as GH hereinbelow). The GH type element is an important type of a liquid crystal display element, and has been attracting attention particularly as means for improving the efficiency of light utilization in color liquid crystal display elements. A variety of GH liquid crystal display elements have been drawing interests, such as those consisting of a laminate of Heilmeier type GH cells, those consisting of DGH (double layer GH) cells, and those consisting of two or three laminated layers of PCGH (cholesteric-nematic phase change type GH) cells. Recently, reflection type liquid crystal display elements are of particular interest in view of energy conservation or securance of battery life in portable devices. The reflection type liquid crystal display elements lack auxiliary light sources such as back lights, so that use of such GH liquid crystal display elements in the reflection type elements is believed to be advantageous due to their high efficiency of light utilization.
As dichroic dyes for the GH liquid crystal displays, excellent materials with high dichroic ratios have been developed, such as azo dyes, anthraquinone dyes, and quinophthalone dyes. Such materials exhibit high dichroic ratios in many liquid-crystalline materials (Alexander V. Ivashchenko, Dichroic Dyes for Liquid Crystal Displays, p165-337, (CRC Press), 1994).
In general, the absorption spectra of dichroic dyes have considerable influence on colored display of GH liquid crystal displays. Thus, in order to broaden the range of the displayable color tones and to ensure color rendering properties, it is necessary to control each of the primaries independently and to combine dyes which overlap little in absorbable wavelength.
The overlap of absorbable wavelengths prevents the independent control of the primaries to narrow the range of the displayable colors, and accompanies additional light absorption to lower the efficiency of light utilization. For avoiding the overlap of absorbable wavelengths, selection of the peak absorption wavelength for each color is important. However, mere selection of the peak absorption wavelength still allows some overlap of absorbable wavelengths since absorption spectra of dyes are generally broad. Thus, it is particularly desired for dichroic dyes to have as narrow light absorption bands as possible in order to reduce the overlap.
Triphenodioxazine was originally developed as a kind of pigments, and it is now generally known to have a narrow light absorption band and to exhibit vivid color tones.
However, the dichroic ratios of conventional dichroic dyes having a triphenodioxazine skeleton are relatively low in liquid crystal, i.e. about 6 to 7 at the best, compared to those of azo dyes and anthraquinone dyes (Alexander V. Ivashchenko, Dichroic Dyes for Liquid Crystal Displays, p173, (CRC Press), 1994). Consequently, development of triphenodioxazine dyes with higher dichroic ratios is desired.